On 24.08.2013 21:44, Mark Morgan Lloyd wrote:
Steve wrote:
> The point with using Linux as a first target is that you would not
need
> to implement a new RTL, but only the code generator and the processor
> specific RTL code.
Hmmm! I think it will be more complex than that... Certainly, anything
related to I/O will need to be drastically rethought, even before it's
rewritten. The I/O architecture is just too different.
It's not. Linux on 390 (caveat: my experience is on Hercules, but I have
no reason to believe that the distro I was using had been tailored for
emulation) looks like Linux on SPARC, Linux on MIPS, Linux on PPC... and
so on. The only significant departure is that they don't see Ethernet
hardware, but instead talk SLIP to a gateway provided by Hercules or use
some sort of offload engine.
Exactly. On Linux we'd simply use the Linux I/O API which is already
implemented in FPC. This is why it's a good idea to use Linux at first:
less work on the RTL.
> The point for gas/ld was simply that we have existing writers for
these
> two, but writing your own writers for IBM specific tools isn't rocket
> science either... But it's another thing you'd need to implement.
You don't have existing writers! They may be familiar with the basic
syntax
that gas/ld use, but IBM assembler, gas, Assembler F or High Level
Assembler
has a totally different vocabulary. The semantics are also unique. CALL
exists but is different, RET doesn't exist. JMP and it's variants are
called
B (B for Branch) and the variants are different. What is the difference
between L and LA? What are L and LA? MOV doesn't exist, there a whole
host
of different instructions for moving data depending upon source,
destination
and format. Do they understand what the 16M line and the 2G bar are and
their implications for code? And a whole other list longer than my
arm. The
manual that defines the Assembler language is, at last count, 1292
pages of
A4! All of it different from what your pool of gas coders is familiar
with,
unless they have previously written IBM Assembler, in which case they
probably used Assembler F or one of it's successors.
STOP. I think that between us we've got a terminology problem: Sven
wasn't using "writer" to refer to somebody writing assembler code, but
to an object embedded in the compiler that is tailored for emitting
assembler statements.
In other words, when we say that an existing assembler writer outputting
gas-format statements for (say) MIPS or 68K can probably be adapted
without too much trouble for s390, we aren't referring to coders on the
development team but to a body of code that comprises a significant
proportion of the compiler's backend.
Exactly. Thank you for explaining. :)
Elaborate enough?
Yes, it was. But Pascal is a nieche language currently, so we need to be
grateful already that we can use C libraries without much problems...
also FPC comes with some header conversions already included, like MySQL
including a database connection wrapper for TDataset ;)
Regards,
Sven
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